Synchronize the code with the guide

src/main/kotlin/com/example/blog/HtmlController.kt

@@ -1,10 +1,12 @@
 package com.example.blog
 
+import org.springframework.http.HttpStatus.*
 import org.springframework.stereotype.Controller
 import org.springframework.ui.Model
 import org.springframework.ui.set
 import org.springframework.web.bind.annotation.GetMapping
 import org.springframework.web.bind.annotation.PathVariable
+import org.springframework.web.server.ResponseStatusException
 
 @Controller
 class HtmlController(private val repository: ArticleRepository,
@@ -23,7 +25,7 @@ class HtmlController(private val repository: ArticleRepository,
 		val article = repository
 				.findBySlug(slug)
 				?.render()
-				?: throw IllegalArgumentException("Wrong article slug provided")
+				?: throw ResponseStatusException(NOT_FOUND, "This article does not exist")
 		model["title"] = article.title
 		model["article"] = article
 		return "article"

src/main/kotlin/com/example/blog/HttpControllers.kt

@@ -13,7 +13,7 @@ class ArticleController(private val repository: ArticleRepository) {
 
 	@GetMapping("/{slug}")
 	fun findOne(@PathVariable slug: String) =
-			repository.findBySlug(slug) ?: throw ResponseStatusException(NOT_FOUND, "Wrong article slug provided")
+			repository.findBySlug(slug) ?: throw ResponseStatusException(NOT_FOUND, "This article does not exist")
 
 }
 
@@ -25,5 +25,5 @@ class UserController(private val repository: UserRepository) {
 	fun findAll() = repository.findAll()
 
 	@GetMapping("/{login}")
-	fun findOne(@PathVariable login: String) = repository.findByLogin(login)
+	fun findOne(@PathVariable login: String) = repository.findByLogin(login) ?: throw ResponseStatusException(NOT_FOUND, "This user does not exist")
 }

src/main/kotlin/com/example/blog/Repositories.kt

@@ -8,6 +8,6 @@ interface ArticleRepository : CrudRepository<Article, Long> {
 }
 
 interface UserRepository : CrudRepository<User, Long> {
-	fun findByLogin(login: String): User
+	fun findByLogin(login: String): User?
 }